1. Field of the Invention
The present invention relates to a step motor of linear type, wherein a shaft is threadedly engaged with an inner hole provided on a rotor and is driven in the axial direction by the rotation of the rotor.
2. Description of the Prior Art
In the prior art, a step motor of linear type, in which a shaft having a male screw is threadedly engaged with a female screw provided on an inner hole of a rotor and is driven in the axial direction by the rotation of the rotor, has been used to drive of a flow rate control valve as disclosed in Japanese patent application laid-open No. 210139/1982, for example.
As shown in FIG. 1, the step motor comprises a housing 31, a stator 32 fixed in the housing 31, a rotor 33 with a magnet member rotatably supported by an inner race of a ball bearing 34, a female screw provided on an inner hole of the rotor 33, and a shaft having a male screw threadedly engaged with the inner hole of the rotor 33 and slidably supported in the axial direction by a front bearing 36 and a rear bearing 37. A coil spring 38 is externally installed to the front end portion of the shaft 35 in order to eliminate backlash of the screw member and prevent chattering of the shaft 35,and the shaft 35 is normally biased to the left in FIG. 1 by the coil spring 38.
In the step motor of linear type in the prior art, however, since the front end portion of the shaft is lengthened and the coil spring is fitted to the lengthened portion, the step motor has structure with the front portion projected and is difficult to be assembled to other devices. Furthermore, during the manufacturing, assembling property of parts including the coil spring is bad and the number of assembling processes increases.